Field of the Invention
Embodiments of the present invention relate generally to a device for generating magnetic field, a method for preparing a device for generating a magnetic field and a gas sensor for measurement of a paramagnetic gas component.
Description of the Prior Art
In anesthesia or in intensive care, the condition of a patient is often monitored, for example, by analyzing the gas exhaled by the patient for its content. For this reason, either a small portion of the respiratory gas is delivered to a gas analyzer or the gas analyzer is directly connected to the respiratory circuit. In the gas analyzer of mainstream type, the whole volume, or at least the main portion of the breathing air or gas mixture, flows through the analyzer and its measuring chamber. The mainstream sensors on the market have mostly measured only one gas, carbon dioxide.
Oxygen (O2) is a gas most vital for life of all living subjects and in healthcare technology there is a firm need to continuously measure its concentration, especially under situations where patients breath artificial gas mixtures with O2 concentrations higher than the 20.9% of the ambient air. This need is most obvious during anesthesia and intensive care. The measurement should be fast enough to be able to record oxygen values during both an inspiratory and an expiratory phase of the breathing cycle. To be able to cover an adequate range of breathing frequencies in children and adults, the response time should be 100 ms or better for both spontaneous and artificial mechanical ventilation. This response time guarantees that, in addition to recording inhaled O2 level, one also gets accurate exhaled O2 level to confirm oxygen delivery to the pulmonary blood. This also makes it possible to calculate O2 uptake, if respiratory flow is also being measured. Traditional electro-chemical O2 sensors have a response time of typically a few seconds. Even if they can be fabricated to achieve faster response times, it is on the cost of their life-time.
Oxygen differs physically from all other relevant respiratory gases appearing in a clinical environment by being strongly paramagnetic. This means that a force is acting on O2 molecules in gradients of magnetic fields. Strictly speaking, this force is proportional to the product of the field strength and its spatial gradient. Generating an alternating field in a gas filled gap of an electro-magnet makes the oxygen molecules in the gap and close to its edges vibrate and generate a pressure signal in synchrony with the current applied to the coil of the magnet. The magnetic field strength is proportional to the flux density divided by the gap length and the pressure signal generated in an oscillating field proportional to the field squared. Even if this effect is macroscopically weak, it has been exploited commercially since the 1970-80's in both industrial and clinical gas measurement applications in various technical configurations.
The most widely used fast response differential paramagnetic O2 analyzer in operation room and critical care application requires measuring in the side-stream set-up, which means the analyzer needs a gas sampling pump and a thin tubing to transport the gas from the patient airway to the measurement cell. This also generates a transport delay of typically 1 to 2 seconds between the signal measured and the real time concentration on the airway. The differential measurement based on this pneumatic bridge set-up also requires a reference gas to be suctioned into the sensor. In most cases, ambient air can be used as a valid reference gas.
There is also known a fast main-stream paramagnetic O2 analyzer requiring no pumping of reference gas. This analyzer comprises an open solenoid type of an acoustic emitter claimed to emit sound with intensity proportional to O2 concentration and a microphone for detection of an amplitude of the propagating sound generated. However, such a solenoid is a problematic component in this application because the dynamic magnetic forces acting on the coil generate an unwanted magneto-mechanical interference signal in phase with the net signal generated by O2.